So I get the idea of bounce testing but am wondering if it's a good idea on really soft sandstone. It seems to me you could blow out a placement by being too aggressive. Additionally a C1 placement in granite is going to hold assuming one is competent to place gear, but should a C1 placement in sandstone be tested? Same for C2, C3 etc.

Bounce testing is always a good idea. That's how hard aid pitches can be made easy. The last thing you want to do is be out multiple placements on a relatively easy aid pitch, or desperate one, (soft rock or not) and zipper a bunch of stuff that you didn't test. I have personally done 100's of aid pitches in desert sandstone of all qualities and only rarely have not bounce tested a piece. It's just prudent, and almost never will "ruin" a placement for future use.